iMac EFI Firmware Update is recommended for all iMac (late 2012) models. This update contains general performance fixes pertaining to sleep and Thunderbolt, and improves compatibility when using the 5GHz band in Wi-Fi. Boot ROM or SMC Version Information: After Update is complete:

Spent about half an hour with Apple support after both of today's updates failed on my 27", 2012 iMac, telling me that neither was supported by the machine. They were surprised at the updates not working and downloaded info from my machine to forward on to engineering. We're scheduled to talk again on Wednesday to see what they've found out about this problem. I told them that there were others besides me experiencing this issue.

Any rhyme or reason we can figure out here? Anyone with a 27 incher that had either or both of today's updates work?

In fact, they updated it in the last 2 hours .... just since I made the post above which was cut/past from the page at that time.

But, I still am not seeing it in Software Update for my 27"... yet.

If I download it from the link (just now) it won't run as it is "unsupported on this machine".

My guess is that the firmware update isn't needed on the 27" models (since they were manufactured a little later than the 21"ers) OR they realized they need a modified update for the 27" machines and are working on one.

This update is recommended for all iMac (late 2012) models. This update contains general performance fixes pertaining to sleep and Thunderbolt, and improves compatibility when using the 5GHz band in Wi-Fi.
Boot ROM or SMC Version Information: After Update is complete:

I just checked the iMac EFI firmware update 2.0 on my iMac 21.5" equipped with fusion drive and the installer declares it as "unsupported". Would be nice to know why Apple did not include this information in their release.